Long wavelength infrared detection and imaging with long wavelength infrared source

ABSTRACT

An infrared detection system comprises the following elements. A laser source provides radiation for illuminating a target (5). This radiation is tuned to at least one wavelength in the fingerprint region of the infrared spectrum. A detector (32) detects radiation backscattered from the target (5). An analyser determines from at least the presence or absence of detected signal in said at least one wavelength whether a predetermined volatile compound is present. An associated detection method is also provided. In embodiments, the laser source is tunable over a plurality of wavelengths, and the detector comprises a hyperspectral imaging system. The laser source may be an optical parametric device has a laser gain medium for generating a pump beam in a pump laser cavity, a pump laser source and a nonlinear medium comprising a ZnGeP2 (ZGP) crystal. On stimulation by the pump beam, the ZnGeP2 (ZGP) crystal is adapted to generate a signal beam having a wavelength in a fingerprint region of the spectrum and an idler beam having a wavelength in the mid-infrared region of the spectrum. The laser gain medium and the ZnGeP2 (ZGP) crystal are located in the pump wave cavity.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of U.S. patentapplication Ser. No. 14/424,933 filed Feb. 25, 2015, which is a 35 USC §371 application of International Application No. PCT/GB2013/052279 filedAug. 30, 2013, which claims priority to Great Britain Patent ApplicationNos. GB 1215423.3 filed Aug. 30, 2012, and GB 1302026.8, filed Feb. 5,2013. The contents of the above-identified applications are incorporatedherein by reference in their entireties.

The present invention relates to a method and apparatus for longwavelength infrared detection and, in preferred embodiments,hyperspectral detection and hyperspectral imaging. It also relates to anoptical parametric device useful in this context. The present inventionis particularly relevant to real time standoff detection, and inparticular detection of volatile substances in real world environments.

BACKGROUND

There are a number of practical applications for remote detection and,if possible, imaging of gaseous species present in a low concentration.These include remote detection of leaks of inflammable or poisonousmaterials and remote detection of explosives. At present, it isdifficult to detect and particularly to image materials remotely insufficiently low concentrations, because the available techniques arenot sufficiently powerful to detect materials in low concentrationsreliably or sufficiently able to discriminate relevant species.

One particularly promising technique is back-scatter absorption gasimaging (BAGI). This technique involves providing a source of lighttuned to a wavelength where the target species has an absorption band,and a detector for detecting light scattered from a target area. Thepresence of gas will occlude an image of a scene from the target areacaptured where there is no gas absorption (for example, at anotherwavelength where there is no absorption from the target species).

It is desirable for the linewidth of the light source to be equal to orless than the width of the absorption band. For short chain hydrocarbonmolecules, absorption bands of interest lie in the 2-4 micron range. Forthese parameter constraints, a particularly suitable light source is anoptical parametric oscillator (OPO) using a nonlinear crystal such asperiodically poled lithium niobate (PPLN). An OPO is a complex opticalsource which comprises a pump laser and a nonlinear crystal. Thenonlinear crystal converts the pump light into two lower frequency (andhence longer wavelength) waves by virtue of a second order nonlinearoptical interaction. The sum of the frequency of these two output wavesis equal to the frequency of the pump input. The lower frequency (andlonger wavelength) output is termed the idler, and the higher frequency(and shorter wavelength) output is termed the signal.

The use of BAGI techniques using OPO light sources has been extensivelystudied at Sandia National Laboratories (SNL). Representative papersfrom this research group include “Backscatter Absorption Gas Imaging—aNew Technique for Gas Visualization” by T. G. McRae and T. J. Kulp,Applied Optics, 1993, 32(21) pp. 4037-4050; “Active infrared imagersvisualize gas leaks” by T. J. Kulp and T. McRae, Laser Focus World,1996, 32(6) p. 211; and “Demonstration of differential backscatterabsorption gas imaging” by P. E. Powers et al, Applied Optics, 200,39(9), pp. 1440-48. Systems using both continuous wave and pulsed OPOsare described, and imaging systems are described including focal-planearray cameras and rastering scanners. However, these systems aregenerally expensive and immobile, and not well adapted to real worldapplications outside a laboratory environment.

A development on this approach is described in WO 2006/061567 A1. Thisdiscloses a BAGI system using an OPO in which the pump wave laser sourceand the nonlinear medium are provided in the same optical cavity. Thisapproach allows for more efficient use of pump laser power, and incombination with use of Q-switching, allows for use in a rapidly pulsedmode which can be used effectively with raster scanning to construct animage of a scene. This makes it possible to produce a less expensive andmore mobile device capable of IR imaging using BAGI techniques.

While these techniques are effective to image the presence or absence ofclasses of material, such as short chain hydrocarbons, they lack theresolution to allow specific materials of interest to be distinguishedfrom a more general class. This is because use of OPOs of this type onlyallows access to the medium wavelength infrared (MWIR) region, typicallydefined as extending from 3-8 μm and shorter wavelengths—for example,the working range of a PPLN OPO is typically from 2-4 μm. This MWIRregion contains absorption bands which are effective to allow a specificclass of material (such a ketone, an unsaturated hydrocarbon or asaturated hydrocarbon) to be recognised, but not to allow one materialwithin that class to be distinguished from another. Recognition ofindividual molecular species typically requires a spectrum over abroader spectral region. Multiple spectral bands, including bands in thelong wavelength infrared (LWIR), typically defined as extending from8-15 μm, can then be used and matched with known or calculated spectrato determine the presence or absence of a particular species. The“fingerprint region” for infrared spectroscopy lies largely in theLWIR—the fingerprint region is normally taken as extending between 500and 1500 cm⁻¹, or 6.67-20 μm. Spectral lines in the fingerprint regiongenerally include relatively sharp lines which result from bendingvibrations specific to the geometry of an individual molecule—thesespectral lines distinguish different members of a class from each otherand can thus be used to identify individual molecular species. ExistingBAGI techniques cannot however work effectively in most of the signatureregion, as known technologies do not function effectively beyond theMWIR region.

SUMMARY

Accordingly, in a first aspect the invention provides an infrareddetection system, comprising: a laser source providing radiation forilluminating a target, wherein the radiation is tuned to at least onewavelength in the fingerprint region of the infrared spectrum; adetector configured to detect radiation backscattered from the target;and an analyser adapted to match detected radiation signals againstpredetermined spectra to determine from at least the presence or absenceof detected signal in said at least one wavelength whether apredetermined volatile compound is present.

This arrangement allows for effective identification of the presence orabsence of specific volatile compounds in remote detection.

Advantageously, the laser source comprises an optical parametricoscillator having a pump laser and a nonlinear medium. Preferably, thenonlinear medium comprises a ZnGeP₂ (ZGP) crystal. This laser sourceprovides good access to the fingerprint region. In one preferredarrangement, the nonlinear crystal is disposed inside a cavity of thepump laser.

In a preferred arrangement, both an idler beam and a signal beam of theoptical parametric laser are provided as output radiation. Preferably,the idler beam provides output radiation at least partly within thefingerprint region and the signal beam provides output radiation atleast partly at shorter wavelengths than in the fingerprint region. Thisis achievable using a ZGP crystal as nonlinear medium.

Preferably the system further comprises tuning means to tune the lasersource between a plurality of wavelengths, wherein the infrareddetection system is a hyperspectral detection system. The analyser maythen be adapted to determine from the presence or absence of detectedsignal in more than one wavelength of the plurality of wavelengthswhether a predetermined volatile compound is present. Moreover, theanalyser may be adapted to determine from the presence or absence ofdetected signal in wavelengths of the plurality of wavelengths whetherone or more of a plurality of predetermined volatile compounds arepresent. In some embodiments a plurality of laser sources are provided,comprising at two optical parametric oscillators with differentnonlinear crystals.

Preferably, the one or more laser sources provide pulsed outputradiation. In a preferred arrangement, the detector comprises an imagingsystem and the infrared detection system comprises an imager, preferablyone that provides an image in real time.

An imaging system is particularly effective in combination with pulsedoutput radiation from the laser sources. The infrared detection systemmay then comprise a scanning system for scanning a target region whereinthe scanning system is synchronised with the pulsed output radiation. Apulse of radiation output by the one or more laser sources at awavelength may then determine an image pixel value at that wavelength.This enables effective hyperspectral imaging.

Preferably, the analyser matches detected radiation signals againstpredetermined spectra to determine the presence or absence of thepredetermined material. Where the detection system is hyperspectral, theanalyser may match detected radiation signals against predeterminedspectra at a plurality of wavelengths determined for that predeterminedmaterial. At least some of the plurality of wavelengths may lie in thefingerprint region. Where the detection system is an imaging system, theanalyser may determine a portion of an image where a predeterminedmaterial is present or absent. The presence or absence of apredetermined material could then be represented in a false colourimage.

In a second aspect, the invention provides a method of determining thepresence or absence of a predetermined volatile compound comprising:illuminating a target with radiation from a laser source tuned to atleast one wavelength in the fingerprint region of the infrared spectrum;detecting radiation backscattered from the target; and determining bymatching detected radiation signals against predetermined spectra fromat least the presence or absence of detected signal in said at least onewavelength whether a predetermined volatile compound is present.

According to a third aspect of the present invention there is providedan optical parametric device having a laser gain medium for generating apump beam in a pump laser cavity, a pump laser source and a nonlinearmedium comprising a ZnGeP₂ (ZGP) crystal, wherein on stimulation by thepump beam, the ZnGeP₂ (ZGP) crystal is adapted to generate a signal beamhaving a wavelength in a fingerprint region of the spectrum and an idlerbeam having a wavelength in the mid-infrared region of the spectrum, andwherein the laser gain medium and the ZnGeP₂ (ZGP) crystal are locatedin the pump laser cavity.

Preferably, the pump laser source comprises a Ho:YAG laser.

Using this approach, an intra-cavity optical parametric oscillator maybe formed using two beam splitter mirrors to separate the signal andidler beams from the pump beam. The beam splitter mirrors may compriseZnSe mirrors.

Preferably, the pump beam is pulsed. In particular, the pump laser maybe Q-switched, for example comprising an acousto-optic Q-switch.

In preferred embodiments, the optical parametric device is adapted fortuning the signal beam and the idler beam over a range of wavelengths.In one arrangement to achieve this, it may further comprise a rotatablemotion stage, wherein the ZnGeP₂ (ZGP) crystal is mounted on therotatable motion stage and the signal and idler beams are tuned byrotation of the rotatable motion stage.

Features described above in respect of the first and second aspects ofthe invention may also apply to this third aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will be described below, by way ofexample, with reference to the accompanying drawings, of which:

FIG. 1 shows the elements of a hyperspectral imaging system inaccordance with an embodiment of the invention;

FIG. 2 shows a LWIR laser source section for use in embodiments of theinvention;

FIG. 3 shows a scanning system for use in embodiments of the invention;

FIG. 4 shows schematically a detection system for use in embodiments ofthe invention;

FIG. 5 illustrates schematically a process for recognising presence orabsence of a predetermined volatile compound; and

FIG. 6 illustrates imaging of a predetermined material using processesdescribed in this specification to illustrate the results of usinghyperspectral imaging for detection.

The main elements of a hyperspectral detection system—in this case, ahyperspectral imaging system, but the imaging aspect can be removed toleave a detection system—are set out in FIG. 1.

A laser source section 1 comprises one or more laser sources forproviding radiation tunable between at least a plurality of wavelengthsfor illuminating a target. As will be discussed below, the laser sourceor sources extend into the LWIR region to allow for use of thefingerprint region of the infrared section. The laser source section 1comprises in this embodiment one or more optical parametric oscillators(OPOs), though embodiments may employ other types of optical sourcetunable in relevant spectral regions. In a preferred arrangement thereis a single OPO used for both LWIR and MWIR operation—in alternativearrangements there may be one OPO for MWIR operation and another OPO forLWIR operation. Each OPO in this embodiment requires a pump laser 11, aQ switch 12 (to enable high speed pulsed operation), a non-linearassembly 13 (as described in more detail below, but comprising anon-linear crystal and a means for moving it with respect to the pumpbeam to achieve tuning) and detectors 14 to measure outputs for use incalibration and control. A laser source control section 2 comprises adrive and temperature control circuit 21 for the pump laser 11, an RFdriver 22 for the Q switch 12, and a stage drive and feedback system 23for the non-linear assembly 13.

The light output by the laser source section 1 reaches the scanning part31 of the scanning and detection assembly 3. The scanning part 31comprises in this embodiment X and Y galvos to produce a raster scan ofa target region. The rasterised output light passes through an objectivelens 4 to reach the target region 5. Unabsorbed light is backscatteredfrom the target region 5 and received through the objective lens 4 atdetectors 32.

The scanning and detection assembly 3 is controlled by an imagercontrolling system 6. This comprises the following: a galvo drive system61 to drive the scanning part 31; detector preamplifiers 62 topreamplify the signals received at the detectors 32 for subsequentsignal processing; a signal conversion system 63 comprising an analogueto digital converter (ADC) operating at suitable speeds (GHz speeds forreal time hyperspectral imaging) and a field programmable gate array(FPGA) to prepare signals for analysis, including providing appropriategating so that detected signals are associated correctly with aparticular position in a raster scan; and a digital signal processor(DSP) 64 programmed to produce hyperspectral images from the convertedand gated signals. The image processing system as a whole may providefurther analysis of the hyperspectral images produced by the DSP 64, theimage processing system as a whole providing analysis to determine fromat least the presence or absence of detected signal in one or morewavelengths within the fingerprint region whether a predeterminedvolatile compound is present.

The system as a whole contains further interface and control elements 7.The laser source control section 2 and the imager controlling system 6are connected to a network processor and interface 71—this allows foruser interaction with the system through a user interface 72. The systemas a whole has additional systems allowing it to operate effectively asa standalone instrument—battery 73, external charging system 74, powermanagement system 75 and thermal management system 76.

Individual sections and subsystems of this embodiment will now bedescribed in more detail. Some features of this embodiment are describedin greater detail in WO 2006/061567 A1, to which the reader is directed.Some features of this embodiment are also found in the Firefly-IR-SCdevice provided commercially by M Squared Lasers Limited—this device,embodying aspects of the technology described in WO 2006/061567 A1,comprises a pulsed MWIR laser system with a scanning accessory forimaging.

FIG. 2 shows a LWIR laser source section 200 for use in embodiments ofthe invention. This comprises an intra-cavity OPO with Q-switching, asused in the Firefly-IR-SC device. There are however several differencesin the arrangement provided here, most particularly the use of adifferent non-linear medium, ZGP (ZnGeP₂).

The non-linear material ZGP is useful for generating tunable light inthe fingerprint region of the optical spectrum (6 to 10 microns). Thisregion contains strong absorption features for many chemical groups. Asis described below, by using ZGP it is also possible, from the samedevice, to generate light in the important 2.5 to 3.5 micronmid-infrared region which contains key absorption in hydrocarbons. Priorart approaches to use of ZGP in an optical parametric oscillatorinvolved pumping at 2 microns using pump lasers such as Ho:YAG. Theoscillation threshold of the resulting OPO is high and large pump lasersare required to make the OPO work.

In embodiments described below, reduction of the pump power required foran OPO is achieved by placing the OPO inside the pump laser cavity. Thisis found to increase dramatically the intra-cavity optical field, makingit possible for the OPO to operate at lower pump powers.

An effective approach to achieving high intra-cavity optical fields isto keep optical losses on mirrors and in transmission of opticalcomponents within the cavity very low. ZGP has a large absorption lossat 2 microns where the Ho:YAG laser operates. This appears to suggestthat placing the ZGP OPO intra-cavity within the Ho:YAG laser will notbe advantageous.

In reality the Ho:YAG has a large gain and, so the system is able tosupport large intra-cavity losses. Even field enhancements of a factorof 2 or 3 make a significant difference in the size and practicality ofthe pump laser required—this can lead to reduction of pump laser powerfrom 20 W to 8 W, makes the choice of components very much easier andreducing the overall cost of the device while increasing flexibility ofdesign.

A further benefit of intra-cavity location is that the pump beam passesthrough the OPO in both directions without the need for opticalisolation between the pump laser and OPO. This makes the devices morecompact, cheaper and lowers the threshold further.

The individual elements of the system shown in FIG. 2 are discussedbelow.

The pump laser in this embodiment is a Ho:YAG laser 201, lasing at 2090nm—this laser type is extensively used for MWIR OPOs. In this case, toprovide enough power to reach the oscillation threshold for longwavelength operation, an intracavity design is used with the nonlinearcrystal located within the laser cavity of the holmium-doped yttriumaluminium garnet (Ho:YAG) laser 201. The Ho:YAG laser 201 is pumped witha commercially available (for example from IPG Photonics Limited)thulium fiber laser operating at 1908 nm—in the arrangement shown, the1908 nm fiber output is first reduced by a 3:1 telescope 202.Alternative diode lasers could be used to reduce the overall size of thesystem. The Ho:YAG laser 201 is Q-switched using an acousto-opticQ-switch 203 (for example, Gooch and Housego QS041-10M-H17). Both theHo:YAG crystal and the Q-switch are water cooled. A switching rate of 20kHz is used in this exemplary embodiment.

The intra-cavity OPO is formed using two ZnSe beam splitter mirrors 204to separate the OPO beam from the 2090 nm pump beam and two curved endmirrors 205, with a radius of curvature 150 mm each. The spot diameterin the ZGP crystal 206 was optimized by moving the cavity mirrors toachieve a match to the pump spot size. Dielectric filters were used toensure that no residual pump light at 2090 nm was detected. The beamsplitters 204 are coated for high reflectivity at 2.8-3.3 μm and hightransmission at 5-9 μm. The end mirror 205 at the output is highlyreflecting at 2090 nm and highly transmitting 5-9 μm.

While an intra-cavity approach is shown here, an external cavity canalso be used in embodiments of the invention. In the external cavitycase, larger pump powers are required to reach threshold for the OPO,but once achieved, large output powers can be expected. In the internalcavity case, as indicated above, the threshold for OPO operation is muchlower and can be achieved with more modest pumps. A low output powerwill often be sufficient for spectroscopic applications. As statedabove, a further advantage to the intra-cavity scheme is that the pumpbeam is double passed through the ZGP crystal. The double pass of thepump ensures that there is signal gain in both directions. Thecombination of the pump enhancement and the double pass gain lowers theOPO oscillation threshold further.

Other choices of nonlinear crystal are possible in this range ofoperation—one suitable material is orientation-patterned galliumarsenide (OP GaAs), others are silver gallium selenide (AgGaSe₂),gallium selenide (GaSe) and silver gallium sulphide (AgGaS₂). Thisembodiment is also arranged as a high repetition rate pulsed system byinclusion of Q-switch 203—as the person skilled in the art willappreciate, continuous wave or less rapidly pulsed systems can beprovided using other component choices.

As is described above, an OPO produces a longer wavelength idler beamand a shorter wavelength signal beam. Either or both of the idler andthe signal beam can be used for spectroscopic purposes. The advantage ofusing both beams is that this can extend the spectral range over which asingle OPO can be used—in the case of ZGP, this allows the use of theidler beam for essentially an LWIR region of operation (in thefingerprint region), with the signal beam providing coverage in the MWIRregion. This is a significant advantage of this approach—using ZGP asthe nonlinear crystal, radiation can be generated in both the 3-4 micronrange and the 5-9 micron range with one system if both idler beam andsignal beam are used. An alternative approach is to use multiple lasersource sections with different nonlinear materials to cover differentranges—these could then be optimised to provide either the signal beamor the idler beam as output, if preferred. A suitable MWIR system wouldbe that described in WO 2006/061567 A1, which uses periodically poledlithium niobate (PPLN) as a nonlinear medium.

A motion stage 207 is provided to move the nonlinear crystal 206 inorder to tune the OPO to different output wavelengths. In the case ofZGP, the approach taken is to rotate the nonlinear crystal to tune it toa different wavelength. Phase matching is achieved in the nonlinearmaterial, which is strongly birefringent, by using the differentpolarization states available—a tuning curve determining signal andidler wavelengths for given pump wavelengths is established forrotational angle of the nonlinear crystal. In the case of ZGP, 15degrees of rotation of the crystal can tune the idler from 5 to 9microns while tuning the signal between 2.7 and 3.8 microns. Motionstage 207 is in this case a rotation stage to which the nonlinearcrystal 206 is bonded—conventional commercially available rotationstages provide sufficient accuracy for this purpose.

In the case of a periodically poled material (like PPLN), the motionstage 207 can be a translation stage, as the tuning is instead achievedby the poling separation, which is varied orthogonally to the directionof travel of light through the material, so translation of the nonlinearcrystal 206 in this orthogonal direction can be used for tuning.

For effective detection of the presence or absence of an absorption inthe target molecule, it is desirable for the linewidth of the outputbeam incident upon the target to be similar to or narrower than thelinewidth of the spectral line resulting from that absorption. Thesystem described provides generally suitable linewidths for thespectroscopy of hydrocarbons in the fingerprint region, but if narrowerlinewidths are required, the laser source section 200 may also beprovided with an etalon.

An exemplary scanning system is shown in FIG. 3. The purpose of thescanning system is to scan the incident beam (whether signal, idler orboth) across the target region in order to produce a rasterized scan,and so construct an image of the target region. The same optics are usedto transmit the incident beam and to capture the backscattered beam fortransmission to the detector system.

In the arrangement shown in FIG. 3, a collimating lens LC with hightransmission in the relevant spectral region (e.g. calcium fluoride)collects the output beam from the OPO and this is directed by mirror mon to a first scanning mirror (in this case, polygonal scanner PS).Light from the first scanning mirror is reflected on to the secondscanning mirror, tilting mirror TM. These two scanning mirrors providetwo axes of the rasterised scan—polygonal scanner PS provides slowerscanning along one (X) axis, whereas the tilting mirror TM driven byhigh speed galvanometer G provides the rapid scan along the Y axis foreach X axis position.

Scanning is synchronised with the pulsed operation of the laser system.Use of Q-switching provides a rapidly pulsed pump laser, and so OPOoutput. Each pulse is sampled to provide a triggering signal, and thetriggering signal is used to define a pixel. The scanning mirrors aresynchronized with the pulsed operation of the laser so that thebackscattered light received in the detection system can be interpretedas pixel data and hence as image data. The imaging system is alsosynchronized with translation of the translation stage 207 for thenonlinear medium, so individual image frames are associated withspecified output beam wavelengths—in this way image frames may beassembled to form hyperspectral images.

Backscattered radiation from the target is incident on the second andfirst scanning mirrors, and is collected by collection lens L—optionallythis may be followed by a filter F to exclude stray light. Thebackscattered light is then received by a detection system, as describedin FIG. 4 below.

FIG. 4 shows a detection system 400 adapted for using both the signalbeam and the idler beam—as the skilled person will appreciate, thisapproach can be used for signal beam or idler beam only simply byremoving the relevant components.

This system uses five detector elements. Pump monitor 401 is anultrafast (20 ps) detector for generating a timing trigger referencesignal for the system. Signal monitor 402 and idler monitor 403 are fast(2 ns) detectors sampling the energy of the signal beam and pump beamrespectively. Signal receive 404 and idler receive 405 are fast (2 ns)low noise detectors that measure the received signal and idlerwavelength energies as reflected from the target. These detectors mayrequire thermoelectric cooling, particularly for longer wavelength use.

The detectors should be chosen appropriately for performance at therequired wavelength. While suitable mirror materials (eg gold) and lensmaterials (eg ZnSe) may be used across a wide IR range, detectors willtypically have a narrower range. However, HgCdTe (also known as MCT) isan effective solution over the 2-15 micron range. InAs is a possiblesolution up to approximately 8 microns. It may also be possible to usemultiple detector ranges in the instrument, for example using InGaAs forMWIR use and HgCdTe for longer wavelengths.

The output pulses from the detectors pass to a circuit board containingfor each detector a controllable gain preamplifier and a Gigasample persecond (Gs/s) analogue to digital converter (ADC) 410. The outputs ofeach ADC goes to one of two field programmable gate arrays 420 (one forsignal, one for idler) that, on receiving the pump monitor triggerpulse, sequentially writes the digital values into memory.

The FPGAs are configured for time of flight according to the round tripdistance determined from the laser to the target and back. Between laserpulses the FPGA integrates the data values in memory, allowing for timeof flight. All other signals are gated out and ignored. Four values foroutgoing pulse energies and received pulse energies at signal and idlerwavelengths result. Pulse-to-pulse energy variations are cancelled outby dividing the received pulse energy by the outgoing pulse energy toprovide two pixel values, wavelength by wavelength. Each pixel, for animaging system, is associated with a position in the target regiondetermined by the scanning system, and can then be assembled into animage for that wavelength or pair of wavelengths. Different image framesmay be provided at different wavelengths by translation of the nonlinearmedium between frames. In this way a plurality of images are built up ata plurality of predetermined wavelengths. The image processing system430 constructs these images and applies any desired image processingalgorithms. The resulting image files may then be sent out through asuitable network connection for any further processing, viewing andstorage.

A process for detection of a predetermined material and preparation of asuitable image is described below with reference to FIG. 5.

The initial output 501 of the hyperspectral imaging system describedabove is a series of images of a scene at different wavelengths—FIG. 6provides an example of this (in this case, from the imaging of a solidsample of hexamine). These predetermined wavelengths will have beenchosen to correspond to spectral features useful for determining thepresence or absence of one or more predetermined materials. Thesefeatures may be the presence or absence of a particular spectralfeature—such as a specific band in the fingerprint region—or therelative intensity of at a series of particular wavelengths (forexample, the relative intensity at a series of closely relatedwavelengths could allow the determination of the slope of a particularlybroad spectral band). The detected imaging results can then be matched502 against reference spectra. This may not be across the whole image—aparticular region (for example, a region providing a signal in aparticular fingerprint region band) may be used to identify an area ofinterest within the image, and only the pixels of the image in this areaof interest may be considered in the matching and subsequent detectionprocess.

The presence or absence of a predetermined material may thus be detected503 by the result of the matching process. This process may be inmultiple stages—for example, MWIR bands may be used to identify a classof material, with specific lines within the LWIR fingerprint region usedto identify specific molecules. As indicated previously, the linewidthof the laser source needs to be sufficiently narrow for effectivedetection of narrow spectral lines of interest. A practical threshold,dependent in practice on the sensitivity required, the tolerance offalse results allowed, and the proximity of confusingly similarmaterials, needs to be established for each material to establishsatisfactory detection.

For an imaging system, the image region over which the predeterminedmaterial has been detected needs to be determined 504. This may bedetermined on the basis of the region of the target image used in thedetermining step 503, but may also be reassessed and regions of thetarget image characterised as containing or not containing thepredetermined material on the basis of a positive identification ofpresence of that material for a part of the image at least. Theintensity of signal in all or part of the spectrum of the predeterminedmaterial may be used to assign a concentration to the material, or anintensity in the image representative of concentration.

The presence of the predetermined material needs to be shown 505 to theuser. As recognition has taken place using a number of spectral bands,it may not be appropriate to do this by using a specific spectral image,but rather by using a false colour image with a specific false colourassigned to the predetermined material. This may be superposed on animage representative of the features of the scene (such as an image inthe visible or the near infrared). The use of different false colourscan then allow a number of different predetermined materials to beimaged in the same user image—in particular contexts (such as the remotedetection of explosive materials), this may be particularly desirable.

I/We claim:
 1. An infrared detection system, comprising: a laser sourceproviding radiation for illuminating a target, wherein the output of thelaser source excludes a broadband output and is tuned to at least onewavelength in the fingerprint region of the infrared spectrum; adetector configured to detect radiation backscattered from the target,wherein the detector comprises an imaging system; an analyser adapted tomatch detected radiation signals against predetermined spectra todetermine from at least the presence or absence of detected signal insaid at least one wavelength whether a predetermined volatile compoundis present; and an imager adapted to provide an image of the targetincluding an indication of whether the predetermined volatile compoundis present.
 2. The infrared detection system as claimed in claim 1,wherein the laser source comprises an optical parametric oscillatorhaving a pump laser and a nonlinear medium.
 3. The infrared detectionsystem as claimed in claim 2, wherein the nonlinear medium comprises aZnGeP₂ crystal.
 4. The infrared detection system as claimed in claim 2,wherein the nonlinear crystal is disposed inside a cavity of the pumplaser.
 5. The infrared detection system as claimed in claim 2, whereinboth an idler beam and a signal beam of the optical parametric laser areprovided as output radiation.
 6. The infrared detection system asclaimed in claim 5, wherein the idler beam provides output radiation atleast partly within the fingerprint region and the signal beam providesoutput radiation at least partly at shorter wavelengths than in thefingerprint region.
 7. The infrared detection system as claimed in claim1, further comprising a tuning mechanism to tune the output of the lasersource between a plurality of wavelengths, wherein the infrareddetection system is a hyperspectral detection system.
 8. The infrareddetection system as claimed in claim 7, wherein the analyser is adaptedto determine from the presence or absence of detected signal in morethan one wavelength of the plurality of wavelengths whether apredetermined volatile compound is present.
 9. The infrared detectionsystem as claimed in claim 1, wherein the one or more laser sourcesprovide pulsed output radiation.
 10. The infrared detection system asclaimed in claim 9, wherein the infrared detection system provides animage in real time.
 11. The infrared detection system as claimed inclaim 9, wherein the infrared detection system comprises a scanningsystem for scanning a target region and the scanning system issynchronised with the pulsed output radiation.
 12. The infrareddetection system as claimed in claim 9, wherein the analyser determinesa portion of an image where a predetermined material is present orabsent.
 13. The infrared detection system as claimed in claim 1, whereinthe analyser matches detected radiation signals against predeterminedspectra at a plurality of wavelengths determined for that predeterminedmaterial.
 14. The infrared detection system as claimed in claim 13,wherein at least some of the plurality of wavelengths lie in thefingerprint region.
 15. A method of determining the presence or absenceof a predetermined volatile compound comprising: illuminating a targetwith radiation from a laser source, wherein the output of the lasersource excludes a broadband output and is tuned to at least onewavelength in the fingerprint region of the infrared spectrum; detectingradiation backscattered from the target, wherein the detecting stepcomprises an imaging step; determining by matching detected radiationsignals against predetermined spectra from at least the presence orabsence of detected signal in said at least one wavelength whether apredetermined volatile compound is present; and providing an image ofthe target including an indication of whether the predetermined volatilecompound is present.